Communication systems using a CAN protocol are known. CAN, or Controller Area Network, is a registered trademark. Communication systems implementing the CAN protocol transmit both a dominant signal and a recessive signal via a transmission line. For example, a dominant signal may be transmitted to indicate that one of the drivers connected to the transmission line is in a drive state and that the transmission line has a low impedance. On the other hand, when the recessive signal is transmitted via the transmission line, all the drivers connected to the transmission line are in a non-drive state, which means that the transmission line has a high impedance. Therefore, a ringing may be generated when transitioning from a drive state to a non-drive state, i.e., when transitioning from the low impedance state to the high impedance state, or when changing from a dominant to a recessive signal. Communication speed affects ringing, with ringing having a greater, more influential effect on high speed communications.
A CAN-FD protocol (Controller Area Network with Flexible Data rate) is an expansion of the CAN protocol. According to the CAN-FD protocol, a CAN frame transmitted and received by the drivers has, or is divided into, an arbitration field and a non-arbitration field. The arbitration field is used for a bus arbitration, i.e. to arbitrate multiple communication signals on a communication line, and the non-arbitration field (i.e., the data field) is transmitted at a higher communication speed than the arbitration field.
The patent document 1 listed below discloses a ringing suppression method in the non-arbitration field for high speed communications, using a drive method that drives the driver to generate a reverse-polarity differential voltage on the transmission line during a recessive signal output. The polarity of the recessive output signal is the opposite of the polarity for the dominant signal. In other words, when the recessive signal is output, the driver is put in a drive state instead of a non-drive state, for lowering the impedance of the transmission line to suppress the ringing.
(Patent document 1) Publication of US 2014/0330996 A1
However, the conventional technique for suppressing the ringing in the above-described patent document 1 is problematic, because it increases the amplitude of the differential signal. That is, in the conventional technique, the normal signal amplitude, or a level difference between the dominant signal and the recessive signal, doubles when the drive method is employed to suppress the ringing, thereby increasing a radiation noise from the transmission line.